It's not necessary to go into the Frankfurt Hbf to go to
Strassbourg. Whenever you want to know details of German
train travel, go to http://bahn.hafas.de (which takes you to
a page with way too long a URL to remember). The page will
be in German so click on "English".

The one way fare, second class, no discounts, is EUR 45. And
that's mostly on ICEs.